1. Field of the Invention
The present invention relates to a method for identifying the current route of paths in telecommunications MS-SPRINGS.
2. Description of the Prior Art
As it is known, the existing ring-shaped optical telecommunications networks comprise nodes which are joined by spans, which spans are equally shared between working channels and protection channels. It is thus possible to protect the information traffic on said telecommunications network by carrying out switching operations between working channels and protection channels. The switching operations are driven by protection words which are exchanged between the nodes of said telecommunications networks.
In current telecommunication networks it has become very important to have the capability of restoring faults possibly occurring in the networks themselves without impairing the functionality of the service. Therefore, the telecommunications networks, and in particular the fiber-optic networks, are provided with means protecting the networks themself against possible failures of network elements (namely, nodes), network spans or network components.
In the MS-SP (Multiplex Section Shared Protection) RING networks, for instance, a shared protection mechanism is implemented, which mechanism allowing the automatic traffic restoration in case of defects or failures in connection fibers. In other words, the MS-SPRING networks perform the automatic traffic restoration through a synchronized re-routing of said traffic, which is carried out at each node of the ring. The operation is controlled by a protocol consisting of 16-bit patterns that are continuously exchanged between adjacent nodes. Said protocol and the operations involved thereby in connection with the different bit patterns are defined by many International Standards, issued by ANSI, by ITU-T and by ETSI, and are characterized by a certain set of rules and messages. See, for instance, ITU-T Recommendation G. 841.
The protection in an MS-SPRING is implemented according to a “Bridge and Switch” technique that substantially consists in re-routing the traffic, through a proper modification of the internal connections of the network elements, switching it from the working capacity to the protection capacity.
The Bridge operation substantially causes a node to transmit the same traffic over both the working capacity and the protection capacity, whereas the Switch operation corresponds to a selection of the traffic traveling over the protection capacity instead of the traffic traveling over the working capacity.
Such a protection technique, requires for each network element be provided, inside thereof, with a device, an APS (Automatic Protection Switch) controller, which is able to detect any line failures, to send and receive information concerning the other network elements by means of said protocol, and carry out the Bridge and Switch actions.
In the four-fiber rings, the fault restoring occurs in accordance with two different ways: in case of failure at the sole working capacity of a given span, the traffic is rerouted over the corresponding protection capacity of the same span (span rerouting), whereas in case of failure of both the working capacity and the protection capacity of a span, the traffic is re-routed over the ring (ring rerouting) in such a way as to travel the alternative route joining the two termination nodes so as to avoid crossing the failured span. Obviously, in two-fiber rings, being the working and the protection capacities both allocated on the same bi-directional fiber pair, only the ring rerouting is applicable.
The Standards define two different types of MS-SPRING protection mechanisms: the classic and the transoceanic algorithms, the latter algorithm being especially fit for networks involving distances between nodes on the order of thousands of kilometers. The two algorithms allow the achievement of the same result in terms of traffic protection still using different rerouting methods.
In accordance with the Standards (see ITU-T G. 774) defining the information content that an SDH network element is obliged to supply to the management system, in case of an MS-SPRING each network element must communicate the current status of its APS controller. The values of this status can be:                the node is requesting the intervention of the protection mechanism to serve an event (namely, a failure or a user command) resulting in a span rerouting; in this case the node is termed “span node” for the sake of conciseness;        the node is requesting the intervention of the protection mechanism to serve an event (namely, a failure or a user command) resulting in a ring re-routing; in this case the node is termed “ring node” for the sake of conciseness;        the node is not requesting the intervention of the protection mechanism (this macro general state may correspond to different elementary states: no events, events of failures in protection spans only, “pending” events, namely those events which can not be served immediately, protocol failure events, condition of intermediate node, namely the condition of a node that declares to accept the request of other span or ring nodes).        
Every ring network span which is affected by an event that the protocol, on the basis of the priorities of all the events existing in the ring, has decided to serve (namely to reroute the protected traffic that has been lost because of the event itself) must always have a pair of span or ring nodes at its two ends.
In any case, for the network manager the problem to be solved is to know, at any time, which paths are currently carried in a given span. The current route of each path can in fact be different from its nominal route if the path itself has been rerouted in order to protect it against failures or it has been rerouted upon commands sent by the user (for instance to perform the maintenance).
For instance, a protected path which, in a failure-free condition is carried on the working capacity of a given span of a four-fiber ring, will be subjected to a span rerouting and carried on the protection capacity of the same span, should a failure affect the working fiber. Hence, by using the present network management systems, the network manager only knows the status of the APS controller of the nodes but it does not know exactly which paths are carried on a given span, at the time when events resulting in the rerouting of the paths occur.
An attempt to solve such a problem consists in a standardized mechanism, called “Path Trace” (see ITU-T G. 707 for the arrangement thereof in the SDH frame and ITU-T G. 783 for the operation aspects) mainly designed to detect misconnections. A further alternative could be that in which the various nodes report the path status to the management system instead of reporting the status thereof. This, however, would be at present possible for transoceanic rings only, wherein the nodes know the entity “path”, but it is impossible for the classic application. Moreover, the application of this mechanism would result in a lot of work to redesign the internal structure handling messages in the SDH apparatus.